1. Field of The Invention
This invention relates generally to emergency warning systems and, in particular, to a control circuit for supplying AC and DC power to a plurality of emergency signs with each sign having a plurality of series and parallel connected light sources.
2. Prior Art
All public buildings are required to have illuminated emergency exit signs indicating emergency evacuation routes. Uniform standards exist as to the exit signs' visibility, light intensity, lettering size, wording, and fail-safe operation. For example, emergency lighting systems must be designed and installed so that the exit sign is continuously illuminated and have an emergency backup power supply should the normal power supply fail.
Many buildings have numerous exits and a single multi-floor structure can contain literally thousands of exit signs. This many exit signs are expensive to install and consume thousands of kilowatts of electricity per year to operate and hundreds of man hours per year to maintain.
Most current exit signs are illuminated by a pair of 20 watt incandescent or fluorescent light bulbs. Since light bulbs burn out over time they must be replaced periodically. To replace the light bulbs a maintenance person must go to each exit sign, get out a step ladder to reach the exit sign, dismantle the exit sign, remove the burnt out bulb, install a new light bulb, reassemble the exit sign, remove the ladder, and discard the burnt out bulb. Just the cost of replacing these bulbs can become quite high.
Many types of the emergency exit signs are shown in the prior art including those that operate on high voltage AC power. Common high voltage systems use anywhere from 100 to 277 volts AC power. Most AC powered exit signs use a battery backup should AC power be lost. The batteries themselves must be regularly inspected and replaced. It has been estimated by those experienced in the art that each emergency exit sign costs between $70 and $120 per year to operate and maintain.
The use of high voltage AC power to energize incandescent or fluorescent light bulbs in emergency exit signs has an inherent problem. Light bulbs get hot during use. If a fire is detected the building's sprinkler system is energized and firemen use water to extinguish the fire. The water will contact the light bulbs causing many of them to break. Thus, when emergency exit signs are most needed they may not be available.
One attempted solution is described in Bowman, U.S. Pat. No. 4,682,147. Bowman's attempted solution is to use light emitting diodes in an exit sign which is powered directly from the building's AC power. The AC power charges a battery during normal operation and upon AC power failure switches to DC operation and pulses battery power to the light emitting diodes causing the exit sign to flash. Unfortunately, this arrangement flashes the exit sign whenever AC power is lost and not only when an emergency situation exists. Therefore, people may evacuate a building when the only problem was a short period of AC power loss. Also, maintenance costs are still substantial since each exit sign has its own battery which must be regularly inspected and replaced.
Other attempted solutions have red down lighting from the underside of the exit sign to assist people in seeing the evacuation route. Most painted warning signs along the evacuation route are written in red letters on white or yellow background so as to be easily readable in normal lighting. Some signs by law must use red, white, and yellow coloring, such as all radiation signs. However, in an emergency situation with AC power lost the normal lighting may not be available. The only light will be from the exit signs. Unfortunately, the red down lighting will wash out the red letters making the warning signs unreadable and exposing the people to unnecessary dangers.
It would thus be an improvement over the prior art of emergency exit signs and control circuits to provide a centrally powered, low voltage emergency sign and control circuit which consumes relatively little power, has a comparatively long life, has down lighting which allows red warning signs to be visible, eliminates bulb change out, reduces battery costs, allows central location maintenance, and flashes emergency signs only when the fire alarm is engaged.